Many agriculture related industries produce biomass waste products. The ginning of cotton is one example. Ginning separates the lint, seeds, and foreign matter from the cotton fibers, and this "cotton gin trash" must then be disposed of. In the past it was often incinerated, but that practice is not acceptable under present air pollution laws. One alternative disposal method is to haul the waste back to the farms it came from and return it to the fields. While this has the advantage of enriching the soil, there are also drawbacks, because cotton gin trash includes chemicals such as herbicides, weed seeds, and disease-causing organisms.
The problem of disposing of cotton gin trash is of major significance. It has been estimated that conventional disposal costs ginners from $6.50 to $9.50 per ton (as of 1983), and that in the state of Texas alone, 1,260,000 tons of cotton gin trash are produced per year (as of 1984).
Cotton gin trash has an energy content of about 6,500 to 7,500 Btu/lb. Cotton ginning requires significant amounts of electrical and heat energy to process cotton. However, the cotton ginning industry is seasonal, operating for only 1000 hours per year on average. Related industries such as cottonseed oil mills and textile mills operate 24 hours per day, seven days per week for over 300 days per year. These two post-harvest industries are usually located near cotton gins, hence transportation costs of fuel (trash) from the gin to a cogeneration plant located at an oil mill or textile mill should be relatively low. In addition, both of these post-harvest processing industries utilize large quantities of saturated steam for processing. The natural gas used to produce this process steam can be expensive. It is possible to divert steam from the turbine in a cogeneration plant having the necessary characteristics to displace the process steam produced with natural gas. It would be attractive to utilize energy in the biomass waste to operate a cogeneration plant that would produce electrical energy and process steam for oil seed and/or textile mill processing. This is especially true in view of the increasing price and uncertain availability of oil, natural gas and electrical energy for agricultural processing in the future. Unfortunately, a number of problems stand in the way of doing this.
Direct combustion of high ash biomass such as cotton gin trash in order to fuel a boiler usually is not satisfactory. It leads to severe ash fouling, slagging, and corrosion, necessitating frequent maintenance.
Gasification, in which solid biomass is reacted with less than stoichiometric oxygen, producing a low molecular weight gas product and a solid product known as char, is much more attractive for use with cotton gin trash. However, researchers have found in the past that the low molecular weight gas containing char when burned results in the same slagging, fouling, and corrosion observed in direct combustion systems. Also, burning the low molecular weight gas tends to produce excessive amounts of nitrogen oxides (NO.sub.x) because the raw cotton gin trash has a high nitrogen content. NO.sub.x emissions are strictly regulated by law in the U.S.
A need exists today for ways to obtain useful energy from biomass waste, both to reduce energy costs and to reduce waste disposal problems.